Van der Waals forces and Hydrogen Bonding.
Temporary instantaneous dipoles induce dipoles in neighbors. Present in ALL molecules.
Strength increases with molar mass (more electrons = higher polarizability).
Attraction between permanent dipoles in polar molecules.
Occurs when H is bonded directly to N, O, or F.
Creates very strong dipole interactions. Responsible for water's high boiling point.
Hydrogen bonds are NOT the intramolecular bonds within a molecule. They are intermolecular forces BETWEEN molecules. They are weaker than covalent bonds.
Noble gases (He, Ne, Ar) are monatomic — they have no bonds at all. Yet they can still be liquified at very low temperatures. What intermolecular force is responsible?
London dispersion forces — the only IMF possible for non-polar atoms. Temporary fluctuations in electron density create instantaneous dipoles. As molar mass increases down the group (He → Xe), these forces get stronger, which is why Xe has a much higher boiling point than He.